Sadly enough, it will probably do less damage than you think. The ECU closely controls the shifting. The transimission selector is pretty much "Drive-by-wire" meaning that the ECU actaully controls the shift, rather than a direct linkage as in older trannys. Mostly, I'd expect you'd just wear out the torque converter from high heat after repeated downshifts. You may want to add an aftermarket tranny cooler if you want descent tranny life.

The most abusive thing you can do to your tranny... as was always the case is to force a double or triple downshift.... such as rolling along at 20mph and stomping on the gas...causing the tranny to drop to 1st gear.